1. Field of the Invention
The present invention relates to diamine compound polymers having a condensed aromatic group, which are applicable to various organic electronic devices such as organic electroluminescence elements, electrophotographic photosensitive bodies, organic thin film transistors and organic semiconductor lasers and are excellent in charge transporting ability and light emission characteristics.
2. Description of the Related Art
Charge transporting polymers represented by polyvinyl carbazole (PVK) are promising materials as photoconductive materials for electrophotographic photosensitive bodies and materials for organic electroluminescent elements as described in references known in the art (for example, see non-patent reference 1). Application of these charge transporting polymers to various organic electronic devices, such as organic thin film transistors and organic semiconductor lasers, is expected. These charge transporting polymers are formed as layers and are used as charge transport materials in the electrophotographic photosensitive bodies and organic electroluminescence elements. Such charge transport materials known in the art include charge transporting polymers represented by PVK, and dispersed low molecular charge transport materials including a charge transporting low molecular compound dispersed in a resin. The organic electroluminescence element is usually prepared by depositing a low molecular charge transport material in a vacuum.
Since various materials may be selected for constituting the dispersed low molecular charge transport materials and high performance materials can be readily obtained, the charge transport material of this type is mainly used in the electrophotographic photosensitive bodies.
While the electrophotographic photosensitive body has been used for high speed copy machines and printers in accordance with high performance of organic photosensitive bodies, current performance is not always sufficient, and more prolonged service life is urgently desired. In view of sensitivity and durability, it is mainstream for this organic photosensitive body to be a stacked type, in which the charge transporting layer is disposed on the outermost surface. This charge transport layer is formed from the dispersed low molecular charge transport material, and charge transport layers with sufficiently satisfactory performance with respect to electrical characteristics can be obtained. However, the low molecular charge transport material is poor in compatibility with a resin component constituting a matrix and the low molecular charge transport material decreases the intrinsic mechanical strength of the resin. Therefore, the charge transport layer provided on the surface of the organic photosensitive body intrinsically has poor mechanical strength and is weak with respect to abrasion.
To solve these problems, introducing an alkylene carboxylic acid ester group into the low molecular charge transport material to improve compatibility of the low molecular charge transport material with the resin component has been proposed (Japanese Patent Application Laid-Open (JP-A) Nos. 63-113465 and 5-80550). However, even though compatibility with the resin is improved, the low molecular charge transport material in which the alkylene carboxylic acid ester group is introduced tends to be difficult to crystallize due to a high freedom of molecular motion of the alkylene carboxylic acid ester group itself. Accordingly, industrial scale production of the low molecular charge transport material in which the alkylene carboxylic acid ester group is introduced is difficult and, because it is difficult to purify this charge transport material to a high degree, purification methods, such as chromatography, are necessary. Moreover, since the alkylene carboxylic acid ester group is electron attractive, mobility of charges tends to be decreased.
On the other hand, a large amount of Joules of heat is generated since the organic electroluminescence element is energized with a current density as high as several mA/cm2. Morphology changes are liable to occur by crystalization of the low molecular charge transport material due to the large amount of heat generated when the dispersed low molecular charge transport material is used for the charge transport material of the organic electroluminescence element. Consequently, undesirable phenomena such as a decrease of luminance and dielectric breakdown are caused, resulting in a decrease of the service life of the element.
It has also been a problem from the view point of efficiency and service life that a material having both a charge transporting ability and a luminous property can be hardly obtained by conventional polymer materials.
On the contrary, the charge transporting polymer is being actively studied since it has a possibility of greatly improving the drawbacks described above.
Examples of such a charge transporting polymer include polycarbonate synthesized by polymerization of a specified dihydroxydiarylamine and bischloroformate (see U.S. Pat. No. 4,806,443), polycarbonate synthesized by polymerization of a specified dihydroxyarylamine and phosgene (see U.S. Pat. No. 4,806,444), polycarbonate synthesized by polymerization of bishydroxyarylamine and bisspirochloroformate or phosgene (see U.S. Pat. No. 4,801,517), polycarbonate from polymerization of a specified dihydroxydiarylamine and bishydroxyalkylarylamine, or bishydroxyalkylamine and bischloroformate, and polyester from polymerization with bisacylhalide (see U.S. Pat. Nos. 4,937,165 and 4,959,228).
Further examples include polycarbonate or polyester (see U.S. Pat. No. 5,034,296) or polyurethane (see U.S. Pat. No. 4,983,482) of arylamine having a specified fluorene skeleton; polyester having a specified bisstyrylbisarylamine as a main chain (see Japanese Patent Application Publication (JP-B) No. 59-28903); and polymers and photosensitive bodies having charge transporting substituents, such as hydrazone and triarylamine, as pendants (see JP-A Nos. 61-20953, 1-134456, 1-134457, 1-134462, 4-133065 and 4-133066, and Proceedings of the 37th Applied Physics Joint Meeting 31p-K-12, 1990)
Examples of applications of the organic electroluminescence element include organic electroluminescence elements using π-conjugate polymers represented by paraphenylenevinylene (PPV; Nature, Vol. 357, 477, 1992), and organic electroluminescence elements using polymers having triphenylamine introduced into the side chain of polyphosphazene (Proceedings of the 42nd Polymer Forum 20J21, 1993).
A lot of attention has been paid to organic semiconductors in recent years as a third semiconductor technology following compound semiconductors. Since organic transistors manufactured by taking advantage of this organic semiconductor technology are flexible, they can be used for low-end mobile information terminals such as electronic paper and printable information tags, and research and development of the organic semiconductor have been actively carried out in recent years.
Furthermore, technologies related to fiber-to-the-home (FTTH), which enables low-cost and large capacity transfer of information to ordinary homes, are being actively studied in the field of communication. Expectations for the organic semiconductor laser as a variety of cheap laser light source as one of these technologies are increasing, and the charge transporting polymer is expected to be applied to the organic transistor and organic semiconductor laser.
While various characteristic such as solubility, film deposition ability, mobility, heat resistance and matching of oxidation potential are required for the charge transporting polymer depending on its application, the properties have been usually controlled by introducing substituents. Since the property of the charge transporting polymer is correlated with the property of the charge transport monomer as a starting material, molecular design of the charge transport monomer is important.
For example, while the monomers as the starting materials of the triarylamine polymer described above are roughly classified into two groups of (1) dihydroxy arylamine and (2) bishydroxyalkyl arylamine, purification of dihydroxy arylamine is difficult since it has a readily oxidized aminophenol structure. Particularly, the compound becomes more unstable when it has a parahydroxy-substituted structure.
Moreover, since the compound has a structure in which oxygen is directly substituted to the aromatic ring, charge distribution tends to be biased due to the electron attracting property of the group, and mobility of the molecule is liable to be reduced.
On the other hand, with respect to bishydroxyalkyl arylamine, although the effect of the electron attracting property of oxygen is canceled with the methylene group, synthesis of the monomer is difficult. Since both bromine and iodine are reactive in the reaction between diarylamine or diarylbenzidine and 3-bromoiodobenzene, the product tends to be a mixture to cause a decrease of reaction yield. In addition, since alkyllithium and ethylene oxide used for substituting bromine with lithium is dangerous and highly toxic, careful handling of these compounds is required.
The π-conjugate polymers represented by paraphenylenevinylene (PPV) described above, and the organic electroluminescence elements taking advantage of the charge transporting polymers having triphenylamine introduced into the polyphosphazene side chain involve the problems of color tone, luminous intensity and durability.
Accordingly, developments of the charge transport materials being easy for synthesis and having high charge transporting ability and excellent light emitting characteristics have been desired for developing the organic electronic devices such as organic electroluminescence elements having high luminance and excellent in safety in repeated uses.
As described above, all the fundamental properties, such as mobility, quantum efficiency, productivity, safety and easy handling, required for the charge transport material for utilizing in various application fields could not be simultaneously satisfied in high level in the conventional charge transporting polymer, because synthesis of the polymer was difficult, stability of the materials is poor, characteristics as the charge transport material such as mobility were poor, and the material is toxic.
Furthermore, the conventional charge transporting polymer has not been suitable for the application to the organic electronic device using the charge transport material such as the organic electroluminescence element.
The present invention is provided in view of the problems as described above. The invention provides a diamine compound polymer having condensed aromatic groups that is able to readily and simultaneously satisfy the fundamental properties required for the charge transport material, for example mobility, quantum efficiency, productivity, stability and easy handling, and is applicable to various organic electronic devices. Further, the invention provides an organic electroluminescence element, electrophotographic photosensitive body, field-effect transistor and semiconductor laser using the polymer.